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    Effect of Na content and hydration on the excitation spectrum of the cobaltite Na_xCoO_2 yH_2O

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    We report on a Raman scattering study on the superconducting cobaltite NaxCoO2⋅yH2ONa_xCoO_2\cdot yH_2O as function of Na content and hydration (x≈\approx1/3, 3/4 and y≈\approx0, 2/3, 4/3). The observed phonon scattering and scattering continua are analyzed in terms of lattice strain due to the structural misfit and disorder. Hydration, due to the intercalation of one or two H2OH_2O layers, releases a part of this strain. Our Raman data suggest a connection between disorder on the partly occupied Na sites, the split off of the a1ga_{1g} level from the other t2gt_{2g} states of Co4+Co^{4+} and superconductivity.Comment: 10 pages, 4 figures, for further information see http://www.peter-lemmens.d
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